In recent years there has been a considerable increase in the power supplied to electric arc furnaces employed for melting metals, passing from unitary powers of 16 MW and 20 MVA to powers of more than 85 MW and over 120 MVA.
These high powers create serious problems of disturbance in the tension (flicker) for the mains supply, and also considerable phase displacements due to the inductive characteristics of the loads.
There are various techniques known to the state of the art which are used to reduce such tension fluctuations and/or to re-phase the inductive loads.
Some examples of these techniques are shown in the article by L. C. Elliott "Electric Power Systems for large Arc Furnaces", WESTINGHOUSE ENGINEER, vol. 29, December 1969, pages 143-149, in the article by Yngve Sundberg "The arc furnace as a load in the network", ASEA journal, vol. 49, 1976, pages 75-87, in the article by Richard F. Dudley "Special Design Considerations . . . ", IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, vol. 33, no. 1, January 1997-February 1997 and in the patent U.S. Pat. No. 3,567,837.
A first technique is to place a stationary inductor on each branch of the feed line of the furnace, with the function of limiting the maximum current absorbed by the furnace.
This solution has the advantage of being simple and economical, but has great limits because it cannot be regulated and cannot therefore be adapted to the variable conditions which occur during the melting cycle.
In co-operation with these stationary inductors, there are re-phasing filters on the medium tension bar which generally consist of a condenser and an inductor in serial connection; these have the function of re-phasing and filtering the harmonics introduced onto the line by the furnace and by the feed control system.
Another solution provides to arrange on the feed line, parallel to the re-phasing filters, a system of inductors which can be inserted by means of thyristors.
When the inductive-type power required by the furnace varies, the thyristors insert the system of inductors into, or remove said system from, the line, so as to compensate this variation.
This adjustment system balances the inductive component, and maintains at a low value the total reactive power employed by the furnace, the reactors and the series of re-phasing filters.
This solution has a certain flexibility and versatility, but it also has the disadvantage that it is very expensive and complex to manage, and that it introduces harmonics into the line.
Moreover, the active power of the arcs of the furnace is adjusted only by varying the height of the electrodes by means of appropriate hydraulic units, trying to maintain the resistances of the arcs themselves constant. This entails long response times and limited accuracy due to the mechanical sizes being acted on.
Another technique is to act directly on the current of the arc of the furnace so as to determine the working point and reduce the disturbances.
With this technique, a saturable reactor is arranged in serial connection on the feed line of the furnace which, when excited by an appropriate continuous current, has the characteristic that it has a low reactance value for small values of current required by the furnace and a high value of reactance for high values of current.
Therefore, once the working point has been set by choosing the polarisation current, the saturable reactor automatically limits the overcurrents, with a consequent reduction of flicker on the mains supply.
This solution has the advantage that it does not need a complicated control system and is potentially valid in that it acts directly on the electric sizes feeding the furnace, limiting drops in tension caused by the unstable functioning of the arcs of the furnace.
However, the inclusion of saturable or dividable inductors on the feed line introduces a further lack of linearity into the system in addition to that caused by the furnace itself, and therefore causes a high number of harmonics to be generated and introduced into the line, with the consequent negative effects.
Moreover, these systems are expensive and burdensome to install and manage.
The present applicant has devised, tested and embodied this invention to overcome all these shortcomings with a simple but at the same time very efficient solution.